Laminates comprising adherent layers of polymeric materials, both filled and unfilled, have been used in diverse applications ranging from electrical insulators to bullet proof shields in banks. In particular, laminates formed by a melt coextrusion process enjoy widespread use as films and containers.
Melt coextrusion is a well known and amply documented technique, see for example Plastic Engineering Handbook, edited by Joel Frados, Van Nostrand Reinhold, fourth edition, pages 197-199. Also see U.S. Pat. Nos. 4,404,547 and 4,336,012 to Koch et al., and 4,403,934 and 4,420,451 to Rasmussen et al., among others. The technique can be used to join plies or layers of different polymers exhibiting different and advantageous properties. Thus laminates having polymer layers which differ as to, for example, melting point (if the layer materials are crystalline), glass transition temperature (T.sub.g), oxygen permeability, impact resistance, flexural modulus, and so forth can be relatively easily fabricated.
In particular, laminates having three or more sheets have been developed wherein the outside sheets have a higher or lower use temperature than an inside sheet. Copending application Ser. No. 710,126 filed Mar. 11, 1985, commonly assigned, discloses cookware made from a laminate comprising at least three sheets made from a thermoplastic resin, an inside sheet made from a thermoplastic resin having a lower use temperature than the thermoplastic resin the outside sheets are made from. Copending U.S. patent application Ser. No. 590,791 filed Mar. 19, 1984, commonly assigned, discloses cookware made from a laminate comprising at least three sheets made from a thermoplastic resin, an inside sheet made from a thermoplastic resin having a higher use temperature than the thermoplastic resin the two outside sheets are made of.
In laminates such as those disclosed in the above copending applications the thermoplastic polymers used to make the individual sheets are generally chosen to provide a laminate which can withstand cooking temperatures while also providing good economics. For this use, achieving a sufficiently high stiffness at cooking temperatures to retain dimensional stability while filled with food is a necessary criterion for acceptable performance. Employing only a high use temperature thermoplastic polymer having a T.sub.g on the order of 350.degree.-450.degree. F. to wholly fabricate an article of cookware would, of course, provide useful cookware, but at a prohibitive cost. A cheaper, lower T.sub.g polymer may be used to wholly fabricate such articles, but at considerable loss in stiffness at high use temperatures, even though the polymer may retain some stiffness if it is crystalline. Such articles generally do not retain sufficient dimensional stability under load bearing conditions, however, such a when a heated container is filled with food. Thus a solution is to use a lower T.sub.g material as a "filler" sheet in a laminate which also comprises high T.sub.g sheets for their stiffening capability under load at high cooking temperatures. Thus, useful items capable of withstanding cooking temperatures can be made by laminating a lower T.sub.g polymer which retains some stiffness at cooking temperatures, such as polyethylene terephthalate, with outside layers of a high T.sub.g polymer which retains good stiffness at cooking temperatures, such as polyarylethersulfone.
When making useful articles from such laminates a significant amount of laminate tends to be wasted in the form of laminate scrap which is trimmed from the articles, for example following thermoforming operations. As a reasonably accurate rule of thumb the cost of a polymer tends to increase with the polymer's glass transition temperature (T.sub.g). Thus the generation of laminate scrap can be costly if the scrap contains high T.sub.g sheets and is discarded.
A few attempts have been made to reclaim scrap and fabricate useful articles therefrom. British Pat. No. 1,552,234 discloses building or insulating material fabricated by uniting, with heat in an existing layer, granulated plastic of a particular particle size and then sintering and compressing the existing layer with at least one further layer material having a smaller particle size, such that a sharp boundary between the layers is not formed. U.S. Pat. No. 4,402,889 to Bonis discloses making a sheet with moisture barrier properties by coextruding outer layers of so-called qualified scrap (i.e., virgin plastic or scrap of one type of plastic only) together with a central layer of so-called unqualified scrap (i.e., unspecified, but having certain compositional limitations) having high enough polyolefin content to provide a moisture barrier.
However, if layers of different polymers in a laminate are melt incompatible, that is if the layers are substantially melt immiscible, tending to exist as separate melt phase, then simply grinding the scrap and attempting to melt process it into useful unitary composite articles generally results in articles of poor quality. Such articles are often brittle, exhibit relatively low impact resistance, and have low binding integrity. Terms in the art such as "cheesy" and "splitty", denoting a propensity to flake apart under relatively mild load or stress conditions, are frequently applied to such articles.
The bulk of the prior art otherwise involves using scrap simply as an economic expedient, either to avoid having to discard scrap altogether or to furnish a cheap source of useable materials in place of virgin plastic. The following are exemplary.
U.S. Pat. No. 4,045,603 to Smith discloses using (unspecified) waste shredded thermoplastic synthetic resin material bits and waste shredded cellulose fiber material bits as low-cost sources from which to fabricate construction materials.
U.S. Pat. No. 4,287,147 to Hungerford discloses a method for utilizing selvage or scrap from polyacrylonitrile film manufacturing operations to form a non-homogeneous material for re-extrusion as a discrete layer in multi-layer film.
U.S. Pat. No. 4,234,663 to Catte et al. discloses incorporating, with the aid of a supplementary adhesive graft copolymer layer, scrap into a multi-layer foil comprising a styrenic polymer layer and an olefinic polymer layer, the scrap being obtained during the manufacture of the multi-layer foil.
U.S. Pat. Nos. 4,476,080 and 4,410,602 to Komoda et al. disclose a process and apparatus for forming films or sheet-like products from thermoplastic resin compositions using conventional extruders even when the resin compositions include reclaimed synthetic resins having relatively low thermal decomposition temperatures.
No prior art of which the inventors are aware, however, discloses using scrap laminate to actually improve the laminate from which it was generated by increasing the laminate's stiffness at use temperature.